With quickly growing of the data storage requirements, optical multi-dimensional storage has attracted great attention.
As opposed to the two-dimensional storage apparatuses like CD (compact disc), DVD (digital video disc or digital versatile disc) and Blu-ray drives, volumetric storage systems record data in three dimensions and increase the storage capacity. Multilayer and holographic data storages are two main proposed technologies for optical volumetric storage [see the reference: G. W. Burr, “Three-dimensional optical storage,” Proceedings of SPIE, Vol. 5225, 20 Oct. 2003, San Diego, Calif., USA].
Normal multilayer data storage uses the disk containing many layers of data, each at a different depth in the medium. Since the laser must travel through other data layers before it reaches the desired layer, the laser interacts with every layer that it passes through on the way to and from the desired layer. These interactions cause noise that limits the available layer number. Holographic storage system stores data throughout the volume of the medium. In the holographic medium, the data are stored by intersecting the object beam, which contains the data, and the reference beam. However, the induced interference patterns consist of very fine fringes, any relative position change greater than a fraction of wavelength between the object or reference beam and the recording medium will cause the fringes to move, and blur holographic recording. Since the disk rotation rate is several thousand revolutions per minute, the fast disk rotation is a fundamental difficulty for creating an ideal hologram in the storage medium.
Apart from the three dimensions in space, the other “dimensions” in which the data can be stored have also been considered for increasing the storage capacity. Generally speaking, the more the “dimensions” are used, the more the data may be stored. Among the proposed four-dimensional storage technologies, one is using three spacial dimensions plus one wavelength dimension [see the reference: S. Kawata and Y. Kawata, “Three-dimensional optical data storage using photochromic materials,” Chem. Rev., 100 (5), 2000, pp. 1777-1′788], and another is using three spacial dimensions plus one polarization dimension [see the reference: R. Hagen and T. Bieringer, “Photoaddressable polymers for optical data storage,” Advanced Materials, Vol. 13, No. 23, 2001, pp. 1805-1810]. Among the proposed five-dimensional storage technologies, one is using three spacial dimensions plus one wavelength dimension and one polarization dimension [see P. Zijlstra, J. Chon and M. Gu, “Five-dimensional optical recording mediated by surface plasmons in gold nanorods,” Nature 459, 2009, pp. 410-413], and another is using three spacial dimensions plus one wavelength dimension and one intensity dimension [see S. Liu, “Optical volumetric storage method and apparatus utilizing multiple tiny Bragg reflectors as storage cell,” Canadian patent application, Application number: 2734440, Filing date: 10 Mar. 2011]. These four- or five-dimensional storages promised higher or at least equal storage capacities than or to the normal multilayer and holographic storages, that is, 1 to 10 TB per disk (TB: Terabytes, 1 TB=1000 GB=1000 Gigabytes), and faster data write, read and erase speeds. However, some of these technologies still utilize the normal multilayer or holographic structures, and so may have the interlayer noise and rotation blurring problems.
The optical storage has unique features, such as the removability of the storage medium, no write/read head crash due to wear, and the potential extreme high storage capacity. Therefore, the new and practicable high capacity optical storage technology is still expected. It is obvious that if the data can be stored in six-dimensions, that is, the three spacial dimensions plus an intensity dimension, a wavelength dimension and a polarization dimension (thus almost all of the physical “dimensions” of light are used), the storage capacity should increase further, and the data write, read and erase speeds should be faster. However, it is a challenge to design a system which can store the values of so many physical parameters in a tiny volume without interlayer noise and rotation blurring. In addition, such system should have a relatively simple structure, so that to get moderate size and reasonable price for becoming a popular commercial product. To my knowledge, no one has designed such optical storage system until now.